| Abstract: | Globular clusters (GCs) are established emitters of high-energy (HE, 100
MeV<E<100 GeV) gamma-ray radiation which could originate from the cumulative
emission of the numerous millisecond pulsars (msPSRs) in the clusters’ cores or
from inverse Compton (IC) scattering of relativistic leptons accelerated in the
GC environment. These stellar clusters could also constitute a new class of
sources in the very-high-energy (VHE, E>100 GeV) gamma-ray regime, judging
from the recent detection of a signal from the direction of Terzan 5 with the
H.E.S.S. telescope array.
We searched for point-like and extended VHE gamma-ray emission from 15 GCs
serendipitously covered by H.E.S.S observations and also performed a stacking
analysis combining the data from all GCs to investigate the hypothesis of a
population of faint emitters. Assuming IC emission as the origin of the VHE
gamma-ray signal from the direction of Terzan 5, we calculated the expected
gamma-ray flux from each of the 15 GCs, based on their number of millisecond
pulsars, their optical brightness and the energy density of background photon
fields.
We did not detect significant VHE gamma-ray emission from any of the 15 GCs
in either of the two analyses. Given the uncertainties related to the parameter
determinations, the obtained flux upper limits allow to rule out the simple
IC/msPSR scaling model for NGC 6388 and NGC 7078. The upper limits derived from
the stacking analyses are factors between 2 and 50 below the flux predicted by
the simple leptonic scaling model, depending on the assumed source extent and
the dominant target photon fields. Therefore, Terzan 5 still remains
exceptional among all GCs, as the VHE gamma-ray emission either arises from
extra-ordinarily efficient leptonic processes, or from a recent catastrophic
event, or is even unrelated to the GC itself. |
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